Got code P0420 today on my 2004 Mustang GT with 67k and as far as I know original 02 sensors. I have an off road x pipe with MIL'S . Long story short had a bad MIL before with bank 1 sensor 2 being lean so I made my own MIL and CEL went away.Learning more about 02 sensors than I wanted with that experience, I just wanted to know if I could check voltage at the rear 02 for atleast .50 volts will deff. tell me if the rear sensor is bad or not ? Meaning will a downstream sensor be dead or be sending low voltage if bad ?

 

You can test the downstream sensor as you described (do it before the MIL Eliminator--I.e. in the wires between the sensor and and MIL-E--but it won't tell you much. With the engine idling the voltage should be vary slowly, once or twice a second, between 0.2V and 0.8V. If it does not vary, or is stuck low then the O2 sensor is bad or the heater is not heating it to it's normal temperature of 900° to 1000°F.

If you disconnect the heater wires the resistance between them should be 3.3Ω or so--if not the heater is bad.

If the heater resistance is 3.3Ω or so then you can bench test the O2 sensor using the procedure described here.

A bad "MIL" (I assume you mean MIL Eliminator) on bank 1, sensor 2 (passenger side rear) would not make the engine run lean, or indicate lean operation. P0420 is also a bank 1 code so check out all possible causes for it as well as testing the sensor.

Or, just reset the code and see if it pops up again--which would be the first thing I would do...

 

Ok , We spoke before and my lean code which the code # eludes me now. Was because one of my MIL eliminators wasn't allowing enough voltage to the computer. So I built one of your home made MIL'S and fixed that. So just learning a little about 02 sensors doing that I was just curious if I could test voltage as a tell tell sign of the sensor being bad

 

Or I could switch the rears and see if it's the sensor or not. LOL

 

A MIL eliminator, working or not, wired into either of the rear O2 sensors will NOT cause the engine to run lean--it cannot as the rear sensors have nothing to do with engine control--all they do is monitor catalyst efficiency.

If something I said indicated a rear sensor, or a MIL Eliminator connected to a rear sensor, could affect engine control then I was not clear in what I said, or what I said was misinterpreted...

 

No you didn't think I'm just confusing you . My OLD code was a P2270 bank 1 sensor 2 signal stuck lean . Thats the code I found out that I had a faulty MIL eliminator. The code now is a P0420 catylist efficiency below thresehold . Thats how I found out one of my MIL eliminators was bad, cause it had to much resistance and wasn't allowing the computer to see voltage. If that makes sense